ci/x86: update to llvm 13
Most CI tests are currently running on LLVM 11 (released over 2 years
ago), which predates some of the GPUs we have in CI and prevents
testing RADV's LLVM backend.
LLVM 13 is known to work for RADV, released almost 8 months ago, and
is already available in most distributions. Fedora 36 is even already
on LLVM 14.
So this commit updates x86 testing on llvm 13.
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
This series is preparing for testing RADV's LLVM compiler backend (see !16841 (merged)).